The present invention relates to optical communications, and, more particularly, to polarization-alternating optical signal generation using carrier-suppressed-return-to-zero CSRZ modulation.
Optical fiber communication distance is limited by signal impairments caused by fiber nonlinearity. One of the important research targets is to mitigate the nonlinear impairment. One method to mitigate nonlinear impairments is to use polarization switching, which sometimes is also called as alternate polarization keying. In this method, polarization of the adjacent symbols is alternated between two orthogonal polarizations. Because adjacent symbols have orthogonal polarization, the nonlinear crosstalk between adjacent symbols is reduced, and therefore nonlinear impairments are reduced. It is of great value to design a method for polarization switching which can achieve the desired nonlinear mitigation, without causing additional impairment or sacrificing the system performance.
The solutions offered so far by others heretofore can be divided into two categories: in the first category, the signal is divided into two arms with equal power. On one arm, the adjacent symbols of the optical signal are modulated with alternating 0 and π phase shifts. Before recombining the two arms together, the state of polarizations of the two arms are made orthogonal to each other. When the two arms are recombined, one symbol has two orthogonal polarization components with the same phase while the next symbol has polarization components with opposite phase. Therefore, adjacent symbols are set into orthogonal polarizations. In some cases imparting opposite phases to adjacent symbols is achieved by using phase modulators. In other cases, the opposite phase is imparted in electro-optic devices by making use of the fact that these devices can have polarization dependent index modulation.
In the second category, an optical switch is used to separate adjacent symbols into different paths. Polarizations of the separated symbols are adjusted to be orthogonal before they are recombined.
Accordingly, there is a need for polarization-alternating optical signal generation using CSRZ modulation that improves upon the prior art.